In recent years, nonaqueous electrolyte batteries such as lithium ion secondary batteries have been developed as batteries with high energy density. The nonaqueous electrolyte batteries are anticipated as power sources of hybrid vehicles or electric vehicles and also as uninterruptible power sources of mobile phone base stations. Therefore, the nonaqueous electrolyte batteries are required to have not only basic characteristics of batteries but also other characteristics such as high-speed charging and discharging performance and long-term reliability. The nonaqueous electrolyte batteries capable of performing high-speed charging and discharging have the advantage that a charging time is considerably short. In hybrid vehicles on which the nonaqueous electrolyte batteries capable of performing high-speed charging and discharging are mounted, power performance can be improved and regenerative energy of power can be efficiently collected.
The above-described high-speed charging and discharging can be realized by rapid movement of electrons and lithium ions between a positive electrode and a negative electrode. Nonaqueous electrolyte batteries of the related art include carbon-based negative electrodes having a negative electrode active material formed of a carbonaceous matter. In batteries using carbon-based negative electrodes, dendrites of metal lithium on negative electrodes may precipitate when high-speed charging and discharging are repeated. Since such dendrites cause electric short-circuit inside nonaqueous electrolyte batteries, there is a probability of heat generation or ignition being caused.
Accordingly, to prevent dendrites of metal lithium from precipitating, batteries using a composite metal oxide as a negative electrode active material instead of a carbonaceous matter have been developed. In particular, batteries using a titanium oxide as a negative electrode active material can perform stable high-speed charging and discharging and have characteristics of a longer lifespan than a carbon-based negative electrode.
However, the potential of a titanium oxide with respect to metal lithium is higher (nobler) than that of a carbonaceous matter. Further, an electric capacity per weight of a titanium oxide is low. Therefore, there is a problem in that a weight energy density is lower in batteries using a titanium oxide.
For example, an electrode potential of a titanium oxide is about 1.5 V based on metal lithium and is higher (nobler) than the potential of a carbon-based negative electrode. Since the potential of a titanium oxide is caused by an oxidation-reduction reaction between Ti3+ and Ti4+ when lithium is electrochemically inserted and desorbed, the potential of the titanium oxide is electrochemically restricted.
For an electric capacity per unit weight, the theoretical capacity of a lithium-titanium composite oxide such as a general formula: Li4Ti5O12 is about 175 mAh/g. On the other hand, the theoretical capacity of a general graphite-based electrode material is 372 mAh/g. Accordingly, a capacity density of a titanium oxide is considerably lower than that of a carbon-based negative electrode. This is because the number of sites absorbing lithium is small in a crystal structure of the titanium oxide.
In view of the above description, new electrode materials containing Ti or Nb have been examined and such materials are expected to have a high charging and discharging capacity. In particular, a titanium-niobium composite oxide represented by a general formula: TiNb2O7 has a high theoretical capacity exceeding 300 mAh/g.
However, in an oxide operating with a noble potential of about 1.5 V vs Li/Li+, such as TiNb2O7, there are problems in that it is difficult to form a surface film and decomposition (that is, a side reaction) of an electrolyte easily continues on an electrode active material or an electrode surface.